Method and system for thinning wafer thereof

ABSTRACT

Embodiments of a method for thinning a wafer are provided. The method includes placing a wafer on a support assembly and securing an etching mask to a backside of the wafer. The etching mask covers a peripheral portion of the wafer. The method further includes performing a wet etching process on the backside of the wafer to form a thinned wafer, and the thinned wafer includes peripheral portions having a first thickness and a central portion having a second thickness smaller than the first thickness. Embodiments of system for forming the thinned wafer are also provided.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth in recent year. Technological advances in IC designand material have produced generations of ICs where each generation hassmaller and more complex circuits than previous generations. In thecourse of IC evolution, functional density (i.e., the number ofinterconnected devices per chip area) has generally increased whilegeometric size (i.e., the smallest component (or line) that can befabricated) has decreased. This scaling down process generally providesbenefits by increasing production efficiency and lowering associatedcosts.

Integrated circuits are formed on semiconductor wafers. Thesemiconductor wafers are then sawed into chips. The formation ofintegrated circuits includes many process steps such as deposition,chemical mechanical polish (CMP), plating, and the like. Accordingly,wafers are transported between different equipment.

There are challenges in fabricating an advanced integrated circuit (IC)involving thinning a wafer.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a cross-sectional representation of a method and a systemfor thinning a wafer in accordance with some embodiments of thedisclosure.

FIGS. 2A-2F shows cross-sectional representations of various stages of amethod and a system for thinning the wafer in accordance with someembodiments of the disclosure.

FIG. 3 shows a bottom-view representation of etching mask used in theprocesses illustrated in FIG. 2A to FIG. 2E in accordance with someembodiments of the disclosure.

FIG. 4A shows a top-view representation of a thinned wafer in accordancewith some embodiments of the disclosure.

FIG. 4B shows a cross-sectional representation of a thinned wafer inaccordance with some embodiments of the disclosure.

FIG. 5A shows a bottom-view representation of etching mask in accordancewith some other embodiments of the disclosure.

FIG. 5B shows a top-view representation of thinned wafer thinned byusing etching mask illustrated in FIG. 5A in accordance with some otherembodiments of the disclosure.

FIG. 6 shows a top-view representation of thinned wafer in accordancewith some other embodiments of the disclosure.

FIG. 7A shows a cross-sectional representation of a system for thinninga wafer in accordance with some embodiments of the disclosure.

FIG. 7B shows a bottom view of a nozzle ring used in the systemillustrated in FIG. 7A in accordance with some embodiments of thedisclosure.

FIG. 8A shows a top-view representation of thinned wafer after wetetching process illustrated in FIG. 7A in accordance with someembodiments of the disclosure.

FIG. 8B shows a cross-section representation of thinned waferillustrated in FIG. 8A in accordance with some embodiments of thedisclosure,

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the disclosure. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the performance of a first process before a second process in thedescription that follows may include embodiments in which the secondprocess is performed immediately after the first process, and may alsoinclude embodiments in which additional processes may be performedbetween the first and second processes. Various features may bearbitrarily drawn in different scales for the sake of simplicity andclarity. Furthermore, the formation of a first feature over or on asecond feature in the description that follows may include embodimentsin which the first and second features are formed in direct contact, andmay also include embodiments in which additional features may be formedbetween the first and second features, such that the first and secondfeatures may not be in direct contact.

Embodiments of the disclosure provide a method and a system for thinninga wafer. In some embodiments, the wafer is thinned by a grinding processby using a grinding wheel. During and after the grinding process,particularly during transportation, the thinned wafer could suffer therisk of breakage and edge chipping. Therefore, a temporary supportingstructure (e.g. tape) is necessary to support the thinned wafer.However, there are additionally operations for applying and removing thetemporary supporting structure. Additionally, scratches caused by thegrinding wheel are formed on the thinned wafer, and as a result anadditional polish process is needed to polish the thinned wafer.

In order to reduce the risk of edge breakage and chipping, embodimentsof the disclosure provide a method and a system for thinning a wafer.FIG. 1 shows a three dimensional representation of a system for thinninga wafer 102 in accordance with some embodiments of the disclosure. Wafer102 has a front-side 102 a (not shown in FIG. 1, referring to FIG. 2A)and a backside 102 b, and the integrated circuits (not shown) are formedon front-side 102 a of wafer 102. The front-side 102 a of the wafer 102is protected during the processes. In some embodiments, a supportingstructure (such as a glass or a tape) may be additionally adhered tofront-side 102 a of wafer 102 to improve rigidity and strength of wafer102.

An etching mask 104 is positioned over backside 102 b of wafer 102. Insome embodiments, etching mask 104 is in a shape of a ring with an outeredge 104 b and an inner edge 104 c and covers a part of a peripheralportion of wafer 102. Etching mask 104 further includes openings 105.Detail descriptions of various embodiments of etching mask 104 will bedescribed later.

A wet etching process is performed to backside 102 b of wafer 102 tothin wafer 102. The wet etching process is performed by positioning anetching supplier 120 (such as a nozzle) over backside 102 b of wafer 102to provide an etchant 122. In some embodiments, etchant 122 flows alonga flowing path 123 in a clockwise direction and is expelled from etchingmask 104 through openings 105. In some other embodiments, etchant 122flows in a counter-clockwise direction (not shown).

Etchant 122 is made of an acidic solution containing acid or alkaline,such as hydrofluoric acid (HF), nitric acid (HNO₃) or combinationsthereof. In some embodiments, etchant 122 is made of an acidic solutioncontaining nitric acid (HNO₃) with a concentration in a range from about50% to about 90%. In some embodiments, etchant 122 is made of an acidicsolution containing nitric acid (HNO₃) with a concentration in a rangefrom about 10% to about 50% mixed with hydrofluoric acid (HF) with aconcentration in a range from about 10% to about 50%. An etching rate ofthe wet etching process varies with the concentration of etchant 122,and therefore the acid concentration of etchant 122 may be adjusteddepending on the expected etching rate according to actual applications.

Since etchant 122 is made of an acidic solution, etching mask 104 shouldbe made of an acid-resistant material. In some embodiments, etching mask104 is made of a ceramic material or polymer(s), such aspolytetrafluoroethylene (PTFE) or Teflon.

FIGS. 2A-2F show cross-sectional representations of various stages of amethod and a system for thinning backside 102 b of wafer 102 illustratedin FIG. 1 in accordance with some embodiments of the disclosure.

Referring to FIG. 2A, an etching support assembly 20 includes a chuck21, pins 23, and a cushion 25. Pins 23 and cushion 25 are disposed onchuck 21. Chuck 21 is used to support and secure wafer 102, and cushion25 is used to protect front-side 102 a of wafer 102. Pins 23 are used tosecure etching mask 104 during subsequent operations. Since cushion 25will directly contact wafer 102, cushion 25 is made of yieldingmaterials (such as polymers).

As shown in FIG. 2A, wafer 102 is held by one or more vacuum wand 27with front-side 102 a of wafer 102 facing down. Afterwards, wafer 102 ismoved to chuck 21 as indicated by the arrow and is placed on cushion 25.

Referring to FIG. 2B, in some embodiments, wafer 102 is positioned oncushion 25, and front-side 102 a of wafer 102 contacts cushion 25.Afterwards, etching mask 104 is placed over etching support assembly 20by one or more vacuum holder 29. Etching mask 104 has holes 104 a whichis designed to house and align pins 23. As shown in FIG. 2B, etchingmask 104 has a width W₁ measured from outer edge 104 b to inner edge 104c. In some embodiments, the width W₁ is in a range from 10 mm to 30 mm.In addition, etching mask 104 is in a shape of a ring and has anextending portion on the ring. The extending portion having a width W₂.In some embodiments, the width W₂ is in a range from 1.5 mm to 10 mm.

As shown in FIG. 2B, etching supplier 120 is also positioned over wafer102. In some embodiments, etching supplier 120 is positioned over acentral portion of wafer 102 to uniformly provide etchant 122 during thesubsequent processes.

Referring to FIG. 2C, etching mask 104 and pins 23 are assembled, andetching mask 104 is secured to etching support assembly 20. In someembodiments, etching mask 104 is held in place by its weight and issecured to etching support assembly 20 by pins 23. In some embodiments,etching mask 104 is secured to etching support assembly 20 by fasteningdevices, such as screws. In some other embodiments, etching mask 104 issecured to etching support assembly 20 by vacuum.

As shown in FIG. 2C, a part of the extending portion of etching mask 104directly contacts peripheral portions of wafer 102. Therefore, theperipheral portions of wafer 102 are protected by the extending portionof etching mask 104 during the wet etching process. The peripheralportions of wafer 102 which are in direct contact with the extendingportion of etching mask 104 are etch exclusion portions and have a widthW₃. In some embodiments, width W₃ is in a range from about 1.5 mm toabout 3 mm. In some embodiments, width W₃ is no larger than 3 mm.

As shown in FIG. 2C, when the wet etching process is performed on wafer102, a mixture 122′, which includes etchant 122 provided by etchingsupplier 120 and removed material of wafer 102, flows on wafer 102 andis expelled from wafer 102 through openings 105 of etching mask 104.

Wafer 102 may be spun by chuck 21 during the wet etching process touniformly spread etchant 122. In some embodiments, the spinning rate ofwafer 102 is in a range from about 50 rpm to 3000 rpm. In someembodiments, wafer 102 is heated to speed up the etching rate of the wetetching process. In some embodiments, wafer 102 is heated to atemperature in a range from about 30 degrees to about 70 degrees.

Referring to FIG. 2D, after the wet etching process, etching mask 104 isremoved from etching support assembly 20 by vacuum holder 29, and thenetching supplier 120 is removed. However, when etching mask 104 isfastened to etching support assembly 20 by fastening devices, etchingmask 104 needs to be disassembled first.

As shown in FIG. 2D, after the wet etching process, wafer 102 in FIG. 2Abecomes a thinned wafer 102′ having peripheral portions 102P with athickness T₁ and a central portion 102C with a thickness T₂. In someembodiments, thickness T₁ is in a range from about 500 um to about 950um. In some embodiments, thickness T₂ is in a range from about 50 um toabout 200 um. In some embodiments, thickness T₁ is equal to the originalthickness of wafer 102. In some other embodiments, thickness T₁ isslightly smaller than the original thickness of the wafer 102. In someembodiments, thickness T₁ is smaller than 700 μm. In some embodiments, adifference between thickness T₁ and thickness T₂ is no larger than 50μm.

Referring to FIG. 2E, thinned wafer 102′ is removed by vacuum wands 27from etching support assembly 20 for subsequent processes, as indicatedby the arrow. As mentioned above, thinned wafer 102′ has thickperipheral portions 102P and thin central portion 102C. Such thickperipheral portions 102P enable thinned wafer 102′ to be handled andtransferred without breaking.

Referring to FIG. 2F, after the wet etching process, a dicing tape 106is adhered to thinned wafer 102′ in accordance with some embodiments. Asshown in FIG. 2F, thinned wafer 102′ is placed on a platen 50, and aprotection layer 107 is used to protect front-side 102 a of wafer 102′from directly contacting with platen 50. Dicing tape 106 is then adheredto backside 102 b of thinned wafer 102′ to assist with the subsequentdicing process. As shown in FIG. 2F, a dicing supporter 52 is also usedto prevent dicing tape 106 adhering to protection layer 107 on platen50. Since thinned wafer 102′ has thicker peripheral portions 102P, it iseasier to transfer thinned wafer 102′ to platen 50. In some embodiments,a dicing process is performed to thinned wafer 102′ with dicing tape 106attached thereon.

FIG. 3 shows a bottom-view representation of etching mask 104 used inthe processes illustrated in FIG. 2A to FIG. 2D in accordance with someembodiments of the disclosure. Referring to FIG. 3, etching mask 104 isin a shape of a ring having outer edge 104 b and inner edge 104 c. Asdescribed previously, width W₁ represents a width between outer edge 104b and inner edge 104 c of etching mask 104, and width W₂ represent thewidth of the extending portion of etching mask 104. In addition,openings 105 of etching mask 104 have a width D₁. In some embodiments,the width D₁ is in a range from about 1 mm to about 10 mm.

FIG. 4A shows a top-view representation of thinned wafer 102′ thinned bythe process illustrated in FIG. 2A to FIG. 2E in accordance with someembodiments of the disclosure. After the wet etching process, thinnedwafer 102′ has thick peripheral portions 102P and thin central portion102C. Thick peripheral portions 102P facilitate the handing of thinnedwafer 102′. The pattern of thick peripheral portions 102P corresponds tothe extending portions of etching mask 104. As shown in FIG. 4A, thickperipheral portions 102P form a non-continuous ring-like structure withopenings along the ring.

FIG. 4B shows a cross-sectional representation of thinned wafer 102′illustrated in FIG. 4A in accordance with some embodiments of thedisclosure. Sidewall portions 102S are between peripheral portions 102Pand central portion 102C. As shown in FIG. 4B, sidewall portions 102Shave a gradually increased thickness from central portion 102C (withthickness T₂) to peripheral portions 102P (with thickness T₁).

As shown in FIG. 4B, a region R₁ between central portion 102C andsidewall portions 102S and a region R₂ between sidewall portions 102Sand peripheral portions 102P have smooth and rounded shapes without anysharp angles or micro-cracks.

If wafer 102 is thinned by the grinding process (which involvesmechanical movements), the edge of the thinned wafer could have sharpangles at its edges, resulting in stress concentration on the thinnedwafer. Therefore, the edge of the thinned wafer could easily break. Incontrast, when wafer 102 is thinned by the wet etching process (which isa chemical process with less mechanical force), the edge of thinnedwafer 102′ does not have sharp angles but are smooth. Since thinnedwafer 102′ does not have sharp angles (which causes stress concentrationon the wafer), chipping and/or breakage of thinned wafer 102′ isavoided.

Compared to the wafer thinned by grinding process, wafer 102 is thinnedby wet etching process described above. The wet etching process involvesseveral advantages.

Firstly, the wafer thinned by grinding process is consistently thin andfragile. In addition, since the wafer thinned by grinding process isbrittle, temporary supporting structures (e.g. a tape) are needed tosupport the thinned wafer. Furthermore, additional operations forremoving the supporting structures are needed. In contrast, thickperipheral portions 102P of thinned wafer 102′ can be used as asupporting structure when handling thinned wafer 102′. Therefore,additional temporary supporting structures are not required, andfabrication processes and cost are reduced.

Secondly, the wafer thinned by grinding process usually has scratchesformed on the thinned wafer. Therefore, an additional polish process(such as a CMP process) is required after the grinding process. However,thinned wafer 102′ has no scratches on its surface after the wet etchingprocess, and the additional polish process is not needed.

Thirdly, the wafer thinned by grinding process has sharp angles at itsedge, resulting in stress concentration. In contrast, stressconcentration of thinned wafer 102′ is avoided for having smooth edges.

Fourthly, when the wafer is thinned by the grinding process, the thinnedwafer tends to be broken during the transferring. In order to preventthe edge of the thinned wafer from being broken, the grinding processand the frame mounting process need to be integrated in an in-lineprocessing system. However, a throughput mismatch issue may be generatedin the in-line processing system. In contrast to the wafer thinned bythe grinding process with the entire wafer having the same thickness,thick peripheral portions 102P of thinned wafer 102′ provides supportduring wafer transferring. In addition, the etching process and theframe mounting process may be performed by stand-alone equipments.

FIG. 5A shows a bottom-view representation of etching mask 104 inaccordance with some other embodiments of the disclosure. FIG. 5B showsa top-view representation of thinned wafer 102′ thinned by using etchingmask 104 illustrated in FIG. 5A. As shown in FIG. 5A, etching mask 104is in a shape of a ring with outer portions 104 d and inner portions 104e. Outer portions 104 d of etching mask 104 form a ring-shape patternwith openings 105 a and inner portions 104 e of etching mask 104 form aring-shape pattern with openings 105 b. Openings 105 a have a width D₂.In some embodiments, width D₂ is in a range from about 5 mm to 20 mm.Openings 105 b have a width D₃. In some embodiments, width D₃ is in arange from about 5 mm to 20 mm. As shown in FIG. 5A, openings 105 a andopenings 105 b are not aligned to each other. In some embodiments,during the wet etching process, etching flowing path 123 is in aclockwise direction, and etchant 122 is expelled from etching mask 104through opening 105 a and 105 b. In some other embodiments, the etchingflowing path is in a counter-clockwise direction (not shown).

As shown in FIG. 5B, since outer portions 104 d of etching mask 104 havea ring-shape pattern with openings 105 a, thinned wafer 102′ also has anouter ring-shape pattern 102D on its peripheral regions. In addition,since inner portions 104 e of etching mask 104 have ring-shape patternwith openings 105 b, thinned wafer 102′ also has an inner ring-shapepattern 102E on its peripheral regions. As shown in FIG. 5B, outerring-shape pattern 102D and inner ring-shape pattern 102E are etchexclusion portions with a width W₃ measured from an inner edge of innerring-shape pattern 102E to an outer edge of outer ring-shape pattern102D. In some embodiments, the width W₃ is in a range from about 1.5 mmto 3 mm.

FIG. 6 shows a top-view representation of thinned wafer 102′ inaccordance with some other embodiments of the disclosure. Referring toFIG. 6, thinned wafer 102′ has a pattern including water drop-likeportions 102F, which are arranged in a clockwise direction. Accordingly,etching mask 104 used for forming thinned wafer 102′ in this embodimentalso has water drop-like portions (not shown). The water drop-likeportions on etching mask 104 may assist with the flowing of etchant 122during the wet etching process. As shown in FIG. 6, the water drop-likeportions of thinned wafer 102′ are etch exclusion portions with a widthW₃ measured from an inner edge to an outer edge of the water drop-likeportions. In some embodiments, the width W₃ is in a range from about 1.5mm to 3 mm.

It is noted that patterns of etching mask 104 are not limited toabove-mentioned patterns, and etching mask 104 may have other designs orpatterns. The numbers of the openings may be adjusted according toactual applications.

FIG. 7A shows a cross-sectional representation of a system for thinningwafer 102 in accordance with some embodiments of the disclosure, andlike elements are identified by the same reference numbers as in FIG. 1and are omitted for brevity. FIG. 7B shows a bottom view of a nozzlering 130 used in the system illustrated in FIG. 7A in accordance withsome embodiments of the disclosure.

Referring to FIG. 7A, a first nozzle 120 a is positioned on the centralregion of backside 102 b of wafer 102, and a nozzle ring 130 ispositioned on the peripheral region of backside 102 b of wafer 102 toperform the wet etching process. Nozzle ring 130 includes a plurality ofsecond nozzles 120 b, as shown in FIG. 7B. During the wet etchingprocess, first nozzle 120 a is used to provide an etchant 122 a, andsecond nozzles 120 b on nozzle ring 130 are used to provide a solvent122 b. Etchant 122 a is used to etch the central region of wafer 102,and solvent 122 b is used to dilute the concentration of etchant 122 aon the peripheral region of wafer 102. Therefore, the etching rate onthe central region of wafer 102 is higher than that on the peripheralregion of wafer 102, and a difference in thickness of thinned wafer 102′will be generated by the different etching rates.

In some embodiments, etchant 122 a is made of an acid solution,including hydrofluoric acid (HF), nitric acid (HNO₃) or combinationsthereof. The solvent 122 b includes water or isopropyl alcohol (IPA). Insome embodiments, the etchant 122 is made of an acid solution includinghydrofluoric acid (HF) with a concentration in a range from about 10% to50% mixed with nitric acid (HNO₃) with a concentration in a range fromabout 10% to 50%. In some embodiment, first nozzle 102 a and nozzle ring130 are connected to liquid suppliers.

FIG. 8A shows a top-view representation of thinned wafer 102′ after wetetching process illustrated in FIG. 7A in accordance with someembodiments of the disclosure. FIG. 8B shows a cross-sectionrepresentation of thinned wafer 102′ illustrated in FIG. 8A inaccordance with some embodiments of the disclosure. As shown in FIG. 8B,after the wet etching process shown in FIG. 7A, thinned wafer 102′ hasthick peripheral portion 102G with a thickness T₃ and central portion102C with the thickness T₄. The difference of the thickness results fromdifferent etching rates on wafer 102 during the wet etching process.Thickness T₃ is smaller than an original thickness of wafer 102 butlarger than thickness T₄. In some embodiments, thickness T₃ is in arange from about 300 um to about 950 um. In some embodiments, thicknessT₄ is in a range from about 50 um to about 200 um. In some embodiments,a difference between thickness T₃ and thickness T₄ is in a range fromabout 100 um to about 900 um. Thick peripheral portion 102G facilitateshandling of thinned wafer 102′. The pattern of thick peripheral portion102G corresponds to that of nozzle ring 130 and has a width W₄. In someembodiments, width W₄ is in a range from about 1 mm to about 3 mm. Insome embodiments, width W₄ is no larger than 3 mm.

As mentioned above, thinned wafer 102′ has a thick peripheralportion(s), such as portions 102P, 102D, 102E, 102F, and 102G, and thethick peripheral portion(s) enable the thinned wafer to be handled andtransferred without breaking. Therefore, additional operations forapplying and removing temporary supporting structures are not required,and the fabrication processes and cost of the thinned wafer aredecreased.

Embodiments of mechanisms for thinning a wafer are provided. The waferis thinned by a wet etching process. An etching mask or nozzle ring maybe used in the wet etching process to form a thinned wafer with athicker peripheral portion(s). The thicker peripheral portion enableshandling and transferring the thinned wafer without breaking the thinnedwafer. As a result, processes can be simplified.

In some embodiments, a method for thinning a wafer is provided. Themethod includes placing a wafer on a support assembly and securing anetching mask to a backside of the wafer. The etching mask covers aperipheral portion of the wafer. The method further includes performinga wet etching process on the backside of the wafer to form a thinnedwafer, and the thinned wafer includes peripheral portions having a firstthickness and a central portion having a second thickness smaller thanthe first thickness.

In some embodiments, a method for thinning a wafer is provided. Themethod includes placing a wafer on a support assembly. The method alsoincludes performing a wet etching process on a backside of the wafer toform a thinned wafer. The wet etching process includes positioning afirst nozzle on a central portion of the backside of the wafer andpositioning a nozzle ring on a peripheral portion of the backside of thewafer. The thinned wafer includes a peripheral portion having a firstthickness and a central portion having a second thickness smaller thanthe first thickness.

In some embodiments, an etching mask used in a method for thinning awafer is provided. The etching mask is in a shape of a ring and is madeof an acid resistant material.

While the disclosure has been described by way of example and in termsof the embodiments, it is to be understood that the disclosure is notlimited to the disclosed embodiments. To the contrary, it is intended tocover various modifications and similar arrangements (as would beapparent to those skilled in the art). Therefore, the scope of theappended claims should be accorded the broadest interpretation so as toencompass all such modifications and similar arrangements.

What is claimed is:
 1. A method for thinning a wafer, comprising:placing a wafer on a support assembly; securing an etching mask to abackside of the wafer, wherein the etching mask covers a peripheralportion of the wafer; and performing a wet etching process on thebackside of the wafer to form a thinned wafer, wherein the thinned wafercomprises peripheral portions having a first thickness and a centralportion having a second thickness smaller than the first thickness. 2.The method for thinning a wafer as claimed in claim 1, wherein the firstthickness is in a range from about 500 um to 950 um, and the secondthickness is in a range from about 50 um to 200 um.
 3. The method forthinning a wafer claimed in claim 1, wherein the peripheral portions ofthe thinned wafer directly contacts a part of the extending portion. 4.The method for thinning a wafer as claimed in claim 1, wherein theperipheral portions has a width in a range from about 1.5 mm to about 3mm.
 5. The method for thinning a wafer as claimed in claim 1, whereinthe extending portion has a width in a range from about 1.5 mm to about3 mm.
 6. The method for thinning a wafer as claimed in claim 1, whereinthe etching mask is in a shape of a ring.
 7. The method for thinning awafer as claimed in claim 1, wherein the etching mask has a plurality ofopenings.
 8. The method for thinning a wafer as claimed in claim 1,further comprising: positioning a etching supplier on the backside ofthe wafer to provide an etchant during the wet etching process.
 9. Themethod for thinning a wafer as claimed in claim 8, wherein the etchantcomprises hydrofluoric acid (HF), nitric acid (HNO₃), or combinationsthereof.
 10. A method for thinning a wafer, comprising: placing a waferon a support assembly; performing a wet etching process on a backside ofthe wafer to form a thinned wafer, wherein the wet etching processcomprises: positioning a first nozzle on a central portion of thebackside of the wafer; and positioning a nozzle ring on a peripheralportion of the backside of the wafer; wherein the thinned wafercomprises a peripheral portion having a first thickness and a centralportion having a second thickness smaller than the first thickness. 11.The method for thinning a wafer as claimed in claim 10, wherein abackside of the nozzle ring comprises a plurality of second nozzles. 12.The method for thinning a wafer as claimed in claim 11, wherein thefirst nozzle is used to provide an etchant, and the second nozzles areused to provide a solvent.
 13. The method for thinning a wafer asclaimed in claim 12, wherein the etchant comprises hydrofluoric acid(HF), nitric acid (HNO₃), or combinations thereof.
 14. The method forthinning a wafer as claimed in claim 12, wherein the solvent compriseswater or isopropyl alcohol (IPA).
 15. The method for thinning a wafer asclaimed in claim 10, wherein the nozzle ring is in a shape of a ring.16. The method for thinning a wafer as claimed in claim 10, wherein thefirst thickness is in a range from about 300 um to 950 um, and thesecond thickness is in a range from about 50 um to 200 um.
 17. Anetching mask used in a method for thinning a wafer, wherein the etchingmask is in a shape of a ring and has an extending portion on the ring.18. The etching mask used in a method for thinning a wafer as claimed inclaim 17, wherein the etching mask is made of ceramic material,polytetrafluoroethylene (PTFE) or Teflon.
 19. The etching mask used in amethod for thinning a wafer as claimed in claim 17, wherein the etchingmask has a plurality of openings.
 20. The etching mask used in a methodfor thinning a wafer as claimed in claim 17, wherein the extendingportion has a width in a range from about 1.5 mm to about 3 mm.